1. Field of the Invention
The present invention relates to carbon nanotube antennas and in particular to higher efficiency carbon nanotube antennas.
2. Description of Related Art
Carbon nanotubes have useful properties as antennas for wireless communications. Specifically, these antennas have a wave velocity that is only 1% of that free space, which means that their physical length is only 1% of that of a metal antenna of the same size in wavelengths. For example, a half wavelength dipole antenna at 900 MHz is only 1.7 mm for a carbon nanotube antenna versus 17 cm for a metal antenna. Thus, in the same physical area the carbon nanotube antenna can have 10,000 times more antennas and one million times more antennas in the same volume. The implementation of large numbers of antennas in a small device is becoming increasingly important as the complexity of multiple-input-multiple-output technology increases in wireless systems.
Carbon nanotubes also have other useful properties over metal such as 1) a current density that is much greater, e.g., 1000 times greater than copper, 2) strength that is much greater, e.g., 100 times stronger than steel by weight, and 3) thermal conductivity that is greater than diamond, e.g., 10 times that of copper. Furthermore, as these carbon nanotube antennas consist of rolled-up graphene sheets, graphene electronics can be easily connected to these antennas.
However, carbon nanotube antennas have very low efficiency compared to metal antennas at typical wireless communication frequencies around 1 GHz, e.g., single walled carbon nanotube (SWCNT) antennas can have efficiencies of 1/1,000,000 or −60 dB. Thus, there can be as much as a 60 dB loss between the power sent to the antenna and the power transmitted, which is power lost as heat, which greatly limits the usefulness of these antennas, even when taking into consideration the reduction in size. This problem is known, with one conventional solution proposed that involves the use of bundles of SWCNT antennas, which increases the efficiency by 30-40 dB, although at an increase in antenna size, but the efficiency is still very low.